This invention relates to a pressure gauge and particularly concerns a pressure gauge which can be adapted to measure the fluid pressure over a wide range with a modification of the structure thereof.
Several types of pressure gauges are used to measure the pressure of fluid in many situations. Generally, in the pressure gauge a metallic elements such as a curved tube or a flexible diaphragm is provided as a sensing element which deforms under fluid pressure. The deformation is changed mechanically or electrically into a calibrated dial reading. For example, an important and widely used pressure gauge is the Bourdon-tube pressure gauge (or spring-tube pressure gauge) in which pressure measurement is based on the deformation by the pressure-producing source of an elastic measuring element. The tube of the gauge of circular or oval cross-sectional shape is closed at one end, and the pressure to be measured is applied to the other end, causing the radius of curvature of the tube to increase. The Bourdon tube usually has limited accuracy due to the inherent structure and material generally used. In another respect, many gears are used as sensing members in conventional devices, and such gauges must be checked from time to time over their full range or scale, making use quite inconvenient for a user. Considering the above shortcomings of conventional devices, the inventor thus discloses a pressure gauge under U.S. Pat. No. 4,040,298 granted on Aug. 9, 1977, which includes neither Bourdon tube nor gears, but rather is comprised of a diaphragm sensitive to pressure, a driving helix and a rotary shaft which performs the function of pressure indication. However, to be a sensitive element, the diaphragm has a rather large surface which is not suitable for measuring a low pressure fluid.